The use of air-release molding has been widely adopted in the ware-forming industry. The most common process involves pressing a quantity of plastic clay between cooperating male and female molds or dies formed of porous, fluid-permeable material. In addition to shaping the ware, the pressing operation also substantially dewaters the clay by forcing excess water into the pores of the molds. Release of the shaped ware, which adheres to the mold faces, is accomplished without distorting or damaging the ware by applying fluid pressure to a fluid permeable conduit communicating with one of the porous mold bodies so that the fluid passes from the conduit, diffuses through the porous mold body, and exits through the mold face as a uniform blanket shortly before the male and female mold members are separated. The shaped clay ware adheres to the other mold member which is transferred to a ware depositing station where fluid pressure is applied in a like manner to the other mold member to complete the release of the formed article. This basic process is disclosed in U.S. Pat. No. 2,584,109 and U.S. Pat No. 2,584,110.
Porous, fired ceramic materials have come into use as fluid-permeable, air-release mold bodies because of their high strength and wear resistance. Such materials are disclosed in U.S. Pat. No. 3,384,499 and U.S. Pat. No. 3,641,229. In practice, it has been found that after such molds have been used for several thousand forming operations, the formed clay articles stick to the molds. The number of pressing operations which can be completed before objectionable sticking occurs varies with the procedures of the particular pressing operation involved and with the composition of the plastic material pressed by the mold. While in some few instances over ten thousand pressings can be effected before sticking will occur, the average number of pressings which can be effected before sticking lies between six thousand and eight thousand forming operations. When sticking occurs, tests show that the fluid permeability of the mold face has decreased to such an extent that it is not possible to secure proper release of the formed clay article from the mold. Once formed articles start to stick to the mold, the mold must either be rehabilitated or it must be discarded despite the fact that the mold face is not appreciabily worn or broken. It has been determined that the decrease in porosity of the molds results from accumulation of material from the clay formed by the mold, in the pores near the surface of the mold body. Qualitative spectrographic analysis of scrapings of the accumulated material indicates, in addition to the presence of alumina from the mold, the presence of potassium and significant amounts of silicon. Photomicrographs of the accumulated material on the surface of a fouled mold show it to be amorphous, having no apparent grain definition or crystalline structure. This suggests that the material is comprised of colloidal particles which were originally dispersed in the plastic clay composition formed by the mold and which were deposited from the clay composition formed by the mold and which were deposited from the clay over the course of repeated forming operations. It is presently believed that the accumulated material principally comprises colloidal silicates from the plastic clay composition formed by the mold.
The accumulation of silicate material from the plastic clay pressed by the mold on the surface and in the pores of the mold body is evidenced by the fact that the surface of the mold face, which initially is light tan in color, assumes a dark, murky brown tone. This accumulation of material so reduces the flow of air out the mold face that release of the formed clay articles from the mold body is impaired and sticking results. This results in a large number of surface imperfections and/or structural flaws due to uneven stresses which in turn produce a large percentage of rejects at the kiln.
Attempts to recondition the molds by washing the accumulated silicate material away with strong acids such as nitric acid, hydrochloric acid or sulfuric acid; with weak acids such as acetic acid; with solvent materials such as kerosene or gasoline; or with weak bases such as ammonium hydroxide or caustic bases such as sodium hydroxide or potassium hydroxide have all been unsuccessful.
Heretofore, the only successful method of restoring the fluid permeability of a used, porous, ceramic mold body which had become fouled with silicates, was to abrade the mold face by sanding or sandblasting as disclosed in U.S. Pat. No. 3,828,488. While such procedures are effective to substantially restore the fluid permeability of the used molds, sanding or sandblasting, even when carried out in the most careful manner, gradually erodes away the surface of the mold face so that after the abrading procedure has been carried out a few times, details of the mold are obliterated, the surface of the mold face is destroyed and dimensional tolerances for the mold are exceeded. Naturally, the exact number of times a given mold can be reconditioned by sandblasting will vary depending on the composition of the mold body, the conditions under which the mold body was fired, the degree of detail in the mold and the required dimensional tolerances for the ware to be produced. Molds used to produce large items which are provided with a heavy glaze, such as toliet tank covers, may be reconditioned as many as five to eight times by careful sandblasting, but fine articles where the surfaces are critical, such as dinnerware, cannot be acceptably produced with molds which have been sandblasted even once. Ordinarily, it may be stated, that the number of times an alumina containing, fired ceramic, porous air-release mold may be reconditioned by sanding or sandblasting will not exceed five or ten times. Thus, after such a mold has been reconditioned just a few times, it is no longer possible to recondition it, and it must be discarded.
If the number of times an alumina containing, porous, fired ceramic mold body could be reconditioned could be significantly increased, significant savings would accrue to the ware forming industry, because the number of expensive replacement molds required would be correspondingly reduced.
Accordingly, it is an object of the present invention to provide a method for reconditioning a porous, fired ceramic mold by removing accumulations of silicate material from the surface and the pores of the mold face.
It is an object of the present invention to provide a method of reconditioning a used, porous, fired ceramic mold body wherein accumulated silicate material is removed by chemical means.
It is a further object of the present invention to provide a method for reconditioning used, porous, fired ceramic mold bodies wherein the removal of accumulated silicate material is effected without significant destruction of the surface of the mold face.